To adapt a hearing aid to hearing loss, required gain curves are calculated on the basis of the measured hearing loss. These prescribe the corresponding gain values for generally three input levels in the frequency range. The ambient conditions when wearing a hearing aid make it possible for the amplified signals output by the hearing aid speaker to be picked up again by the hearing aid microphone. This is especially the case for open supply and with gaps in the seal in the case of closed supply. If the feedback loop is not attenuated, whistling occurs.
The feedback whistling not only disturbs the wearer of the hearing aid but also other people in his or her immediate environment. To avoid the whistling, the nominal gain curve is usually reduced somewhat. To this end the gain curve of the feedback loop is measured according to its separation at a point and, for each frequency, that gain which represents the limit for feedback or feedback whistling is determined. To prevent whistling the values must always be below this limit gain. Since feedback paths are only static under some conditions and thus the limit gain can be temporally exceeded, the gain is generally reduced to the point at which there is always a minimum distance to be measured from the limit gain curve.
The prior art described in the two paragraphs above can for example be found in publication DE 101 31 964 A1. To avoid whistling the publication recommends a method for operating a hearing aid in which a gain reduction is undertaken as soon as an interference noise is recognized as such.
In publication DE 101 59 928 A1 a method for avoiding feedback-related oscillations in a hearing aid is described. When feedback-related oscillations are detected, the gain is reduced in an area of low signal level of the input signal and, in an area of high signal level of the input signal, the gain is reduced less or is not reduced.
Despite reduction in the required gain curve as known in the art, feedback whistling can still occur, especially in the area of resonances, since for example the resonant frequencies can change dynamically depending on the ambient conditions.